How do I know if my dog is pregnant

Hi i am new to this site, this is my 1st time on here, and I am afraid I may have had an accidental breeding. I have a 3 year old Jack Russell/ Rat Terrier mixed male that for medical reasons cannot be neutered and my 15 year old son brought home a Chorkie puppy, we were told she was 5 months old and scheduled her a vet appointment for shots and to have her spayed. Well much to my dismay, when we got to the vet, we were told she was in heat and we would need to wait to have her spayed. So home we went and I kept the dogs separated, not that my male wanted anything to do with her anyways. Three weeks or so in, and it's a totally different story, he's whining and shaking and marking all over our new house. Now here comes the part where I am scared a breeding happened. I was in the shower, the female was upstairs with my children, my amle was locked in my room on the main floor and my sister and her 3 kids come in town for a visit, my 2 year old niece opens my door looking for me, out runs Rhett and follows the older niece and nephew up the stairs and into the room with the female. The kids say the dogs didn't mate, but the kids that were present are only 11 and 12. So here's my question, how long do I need to wait to go to the vet and see if she is in fact pregnant? They have been totally apart before and since that one very brief encounter (and yes I am aware it only takes a few seconds).